1. Field of the Invention
This invention relates to a receiving apparatus in communication equipment using a multicarrier modulation method, and more particularly to a receiving apparatus in communication equipment using the OFDM (Orthogonal Frequency Division Multiplexing) modulation method.
2. Description of the Related Art
OFDM (Orthogonal Frequency Division Multiplexing) is characterized by high bandwidth efficiency and robustness against multipath environment. In recent years, OFDM system has been applied to terrestrial digital television broadcasting and wireless LAN, and therefore has attracted considerable attention.
In the OFDM method, data is allocated to a number of orthogonal subcarriers and then modulation and demodulation are performed. The transmitter requires an IFFT (Inverse Fast Fourier Transform) process and the receiver needs an FFT process. As a result, the configuration of an OFDM transmitting and receiving unit is very complex. However, recent advances in LSI technology have made the configuration feasible.
FIG. 1 shows an example of a multicarrier modulation signal transmitting apparatus.
A channel coding section 1 subjects the transmission data to channel coding process. The channel coding process includes, for example, an error detection process using CRC (Cyclic Redundancy Check) codes and an error correcting process using convolutional codes.
An interleaving section 2 disperses burst errors, randomizes error series, and changes the order of data items to yield a more effective result of the error correction. After a serial/parallel converter converts the transmission data into a symbol string made up of a number of subcarriers, the resulting data is inputted to a mapping section 3.
The mapping section 3 separates the inputted data into the I (real number) component and the Q (imaginary number) component according to a modulation method, such as PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation), thereby determining the amplitude and phase of the subcarriers. The I component corresponds to the real part of a complex number on the frequency axis and the Q component corresponds to the imaginary part of the complex number on the frequency axis.
At an IFFT (Inverse Fast Fourier Transform) section 4, the I component signal and Q component signal in frequency domain are transformed into time domain data. Those signals are further converted into serial data by a parallel/serial converter. A GI (Guard Interval) add section 5 adds a guard interval to the transmission data for the purpose of alleviating interference from delay paths.
The guard-interval-added transmission data is subjected to a filtering process at an FIR (Finite Impulse Response) section 6. Furthermore, an IQ modulation (or orthogonal modulation) section 7 subjects the transmission data to orthogonal modulation.
At a multiplier circuit (or mixer) 8, the transmission data is converted to a radio frequency band using a clock signal generated by a local oscillator 12A. A power amplifier 9 drives an antenna 10A on the basis of the output data from the multiplier circuit 8. The antenna 10A transmits an OFDM signal.
FIG. 2 shows an example of a conventional multicarrier modulation signal receiving apparatus.
The OFDM signal received by an antenna 10B passes through a low-noise amplifier 11, a multiplier circuit (or mixer) 3, and an AGC (Auto Gain Control) circuit 14 and is inputted to an IQ detecting section 15. The frequency of the received data is determined by the clock signal generated by a local oscillator 12B.
The IQ detecting section 15 detects the I (real number) component and Q (imaginary number) component from the received OFDM signal. A loop composed of the IQ detecting section 15, AFC (Auto Frequency Control) circuit 16, and oscillator 17 adjusts the frequency of each of the I component and Q component.
A GI (Guard Interval) removing section 18 removes the guard interval added on the transmission side. An FFT (Fast Fourier Transform) section 19 transforms the time-domain received data (I component and Q component) into frequency-domain data. The received data (I component and Q component) outputted from the FFT section 19 represents the phase and amplitude of each subcarrier of the OFDM signal.
The received data (I component and Q component) outputted from the FFT section 19 is inputted to an equalizing and error processing section 20. The equalizing and error processing section 20 is composed of an equalizing section 21 and an error processing section 22.
Each subcarrier of the OFDM signal is inputted to the equalizing section 21. The equalizing section 21 equalizes each subcarrier. The equalizing section 2 is composed of a channel compensation section 23 and a phase rotation correcting section 24 as shown in FIG. 3. The channel compensation section 23 compensates for the channel distortion. The phase rotation compensation section 24 compensates for the rotation of the phase caused by the frequency offset or the clock difference between the transmitting apparatus and the receiving apparatus.
Each subcarrier subjected to the equalizing process is inputted to an error processing section 22. The error processing section 22 performs an error correcting and detecting process according to the channel coding process carried out on the transmission side. When the transmission side has performed error correction coding and error detecting coding, the error processing section 22 is composed of an error correcting section 25 and an error detecting section 26 as shown in FIG. 4.
The error correcting section 25 corrects correctable errors. The error detecting section 26 detects errors which could not be corrected at the error correcting section 25. When detecting no error, the error detecting section 26 determines that the receiving process has succeeded. When detecting an error, the error detecting section 26 determines that the receiving process has failed.
For example, in a 5-GHz-band wireless LAN, Viterbi decoding is used in the error correcting process at the error correcting section 25 and error detecting using CRC (Cyclic Redundancy Check) codes is performed in the error detecting process at the error detecting section 26.
As shown in FIGS. 2 and 3, in a conventional multicarrier modulation signal receiving unit, the characteristic of the channel is compensated at the channel compensation section of the equalizing section 21 and the rotation of the phase is compensated at the phase rotation compensation section 24 of the equalizing section 21.
When the distortion of the transmission data is caused by the transmission-side power amplifier (9 in FIG. 1), the equalizing section 21 cannot correct the distortion (or power amplifier distortion). Since the OFDM communication method uses the transmission data obtained by multiplexing many subcarriers, the peak to the average power ratio is great, with the result that there is a strong possibility that power amplifier distortion is caused by the transmission-side power amplifier.
When there is a power amplifier distortion, interference between subcarriers takes place, resulting in a more frequent failure in receiving process at the receiving apparatus, which leads to the deterioration of communication quality.
On the other hand, the power amplifier distortion can be decreased by increasing the amount of input back-off of the transmission-side power amplifier. A power amplifier with a large amount of input back-off has the drawback of consuming a large amount of electric power. In this case, the greater part of the power consumption at the transmitting apparatus is attributable to the power amplifier.